1. Field of the Invention
The present invention relates to an electrostatic charging system for electrophotographic copying machine, and particularly to a system in which accuracy of detection is elevated in respect to the current flowing through a photoreceptor by means of corona discharge.
2. Description of the Prior Art
In general, an electrophotographic copying machine is provided with a plurality of corona discharging devices effecting suitably charge, transfer, separation, discharge and the like relating to copying steps in the vicinity of the surface of the photoreceptor such as photoreceptor drum or the like.
In such corona discharging devices, the most important parameter is so-called photoreceptor current flowing through the photoreceptor by means of corona discharge. It is a very important requirement for obtaining a copy image of good quality that such photoreceptor current is exactly measured to adjust or control, for example, discharge output.
However, in conventional electrostatic charging systems for electrophotographic copying machine, accurate photoreceptor current could not be detected, because it is affected by leakage current being generated in a power supply circuit for the corona discharging unit, particularly in the case where output of the power supply circuit is a.c. output. More specifically, in such a system, leakage current is generated through stray capacitance between a ground frame and a high voltage cord for transmitting such a.c. output as mentioned above to the corona discharging device, or through internal capacitance of a high voltage power transformer (hereinafter referred to simply as "transformer") itself for the power supply circuit, and further the leakage current thus generated joins with the above-mentioned photoreceptor current, whereby the accurate detection of the photoreceptor current has been prevented. The leakage current produced from the high voltage cord can be suppressed by such a manner that the high voltage cord is either shortened or removed by directly connecting the transformer with the corona discharging device. However, a countermeasure could not be taken against the leakage current being generated from the transformer itself.